Described herein is a magnetic recording medium comprising a substrate and a magnetic recording layer thereon comprising a radiation cured binder and magnetic particles, the binder containing a blend of a thermoplastic polymer or polymers and a polyfunctional acrylate derivative of a caprolactone-polyol or a polyester polyol.
Magnetic recording media are in the form of magnetic cards and disks, reels, video tapes, computer tapes, etc. Magnetic tape, for example, consists of a very uniform magnetic coating which is bonded to a plastic support film. Magnetic tape is utilized for audio, video, computer, instrumentation, or other recordings.
The basic components of a magnetic tape are the plastic support film and the magnetic coating. The magnetic coating contains magnetized particles which store the information and a resinous binder which provides the cohesive matrix between the magnetic particles and adheres them to the support film. The preferred polymer for the support film is polyethylene terephthalate due to its excellent dimensional stability, high tensile strength, toughness, pliability and resistance to attack by mildew and fungus. However, other polymers such as polyvinyl chloride and polypropylene have been used. Gamma ferric oxide is the most widely used magnetic particle.
Binder systems for magnetic tape coatings are based on blends of hard and plasticizing or toughening resins. The combination of a rigid polymer and an elastomeric polymer offer performance advantages which cannot be obtained with a single component or other simple resin system.
Most conventional binders contain a blend of a hydroxylated copolymer resin, and an elastomeric modifier. The copolymer is usually modified with from 30% to 60% by weight of a plasticizing or toughening resin to increase its abrasion resistance. The amount of modifier used in a formulation is a function of the performance requirements of the particular application. The modifying resin may be a nitrile rubber, a polyester, an alkyd or a polyester urethane. The latter resin is preferred for high performance applications. The polyester urethanes are the reaction products of polyester polyols, short chain diols, and isocyanates. These resins have excellent toughness and abrasion resistance.
Both the hydroxylated copolymer resins and polyester urethane resin contain hydroxyl functionality. The blends of these resins are crosslinked with polyfunctional isocyanates to further increase their toughness and abrasion resistance. Crosslinking of the resins occurs over a period of time including while the tapes are in storage. If the crosslinking takes place prior to calendering, the tape does not polish properly.
For the highest performance applications, such as computer tapes, phenoxy resins are preferred as the hydroxyl containing polymer because of their superior durability, toughness, and thermal stability. The phenoxy resin can be modified with the same polyester urethane plasticizing resins and isocyanate prepolymer crosslinkers or polyfunctional isocyanates as the copolymer resin.
However, curing of polyurethane-based coatings is accomplished with the addition of various multifunctional isocyanates. Once the isocyanate is mixed with the coating formulation, the crosslinking reaction begins immediately, giving the coating a limited pot life on the coater or during the coating/calendering processes. Another disadvantage is the extreme reactivity of the isocyanate molecule with water. A trace of water in the solvent, or moisture absorbed on the magnetic particle, or even moisture in the plant environment will quickly react with the isocyanate negating its ability to crosslink the polyurethane. The resulting film will fall far short of designed toughness and flexibility.
Controlling the many sources of moisture is difficult so nonuniform batch-to-batch cures, are unavoidable. In an effort to overcome this problem, the coated and calendered tape is often stored for two or more weeks; sometimes at elevated temperatures under dry conditions to assure a complete cure. Despite this, it is not uncommon to reject substantial quantities of uncured tape after postcure.
Attempts have been made to formulate binder systems which could be cured by electron beam radiation. Electron curing has been shown to provide a thorough, consistent cure, not possible with the polyurethane/hydroxylated polymer/isocyanate systems.
Experimenters have found they could formulate electron beam cured coatings with resin systems consisting of simple blends of commercially available polyurethanes plus electron curable multifunctional acrylates and methacrylates derived from simple diols, triols and tetraols not containing repeating polyester units. These multifunctional acrylate and methacrylates include trimethylolpropane triacrylate, tripropylene glycol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, trimethylolpropane dimethacrylate, and the like. These blends showed good binding of the magnetic particles, adequate adhesion to polyester film and elimination of blocking (lifting of the coating on reeled tape). However, the mechanical properties of the tapes were poor due to the use of multifunctional acrylates such as trimethylolpropane triacrylate and pentaeythritol triacrylate which embrittled the film. Also, the surface wear resistance of the resulting tapes was variable. Further, these acrylates were volatilized during the solvent drying step when solvents such as cyclohexanone, tetrahydrofuran, etc. were removed. The result was that systems employing these acrylates did not meet the standards of the magnetic media industry.
Thus, there is a need to develop binder systems which are suitable for electron beam curing and which can improve the wear characteristics of the magnetic coating.